A soft fork is a type of upgrade to a blockchain protocol that allows for the introduction of new features or changes without requiring all nodes in the network to upgrade. Unlike a hard fork, which creates a permanent divergence from the existing blockchain, a soft fork is backward-compatible, meaning that nodes that do not upgrade can still interact with upgraded nodes. This characteristic makes soft forks a favored method for implementing changes in blockchains like Bitcoin, where consensus among nodes is crucial for maintaining network stability.